Diffusers for centrifugal compressors are generally characterized as having two confining walls known as the hub wall and the shroud wall. It is known that providing vanes in the diffuser can improve efficiency. Full vanes, or vanes which extend fully between the shroud wall and hub wall have been utilized. Slotted wall type diffusers where full vanes are accepted by slots in one of the walls are also known. Ribbed vanes, or vanes which do not extend fully between walls are known to be advantageous. They function by aligning boundary layer flow with the core flow so that the core flow is not disturbed at the design mass flow.
At partial mass flow, a fixed wall vaneless diffuser is less effective that at design mass flow, whereas a ribbed wall is more effective. Variable wall designs such as that seen in U.S. Pat. No. 3,365,120 issued Jan. 23, 1968 to Jassniker have been employed as solutions to the partial mass flow problem. Slotted wall designs using full vanes are effective but require elaborate mechanical hardware such as a precision slotted wall, anti-rotation device, elaborate sealing, springs to retain the vanes and a pocket behind the slots to contain flow leakage. Such a slotted wall mechanism is shown in U.S. Pat. No. 4,403,914 issued Sept. 13, 1983 to Rogo et al.